1. Field of the Invention
The present invention concerns a computed tomography apparatus of the type having a gantry with at least one x-ray tube and at least one detector, the x-ray tube and the detector being rotatable around a system axis (z-axis), and the detector exhibits a width B in the system axis direction and a length L in the circumferential direction, and furthermore a diaphragm with at least one movable element disposed in front of the detector and thus partially covers the detector as needed.
2. Description of the Prior Art
It is generally known that the resolution can be improved in an x-ray detector by a reduction of the size of the aperture which, among other things, describes the opening ratio or the power of an optical system, as described in German OS 102 11 948, for example. In this document, a diaphragm is described that has a movable element that at least partially covers the surface of the detector elements.
In modern x-ray computed tomography apparatuses, generally multi-row detectors are used that are arranged next to one another in the direction of the system axis (z-axis). Each detector row is formed by a number of detector elements in a direction perpendicular to the z-axis. To improve the resolution of these detectors, as needed a detector diaphragm can be attached in front of the detector that reduces the effective surfaces of the individual detector elements.
Newer developments in detectors exhibit an ever-greater width in the z-direction, which also requires a diaphragm that is wider in the z-direction. Since this detector diaphragm is moved over the detector only as needed, it must otherwise be disposed outside of the beam path in a standby position. This diaphragm is always moved, as is conventional, over the detector in the z-direction. This movement path requires a large amount of space in the z-direction. The gantry housing must accordingly also provide corresponding storage (stowage) space for the detector diaphragm, and is therefore construed correspondingly deep in the z-direction.
This design depth, however, causes a limitation of the collision diagram outline (i.e., the area in which a risk of collision exists), and thus a limitation with regard to the tilt capability of the gantry and with regard to the detector in general also arises. If one wants to obtain a sufficient tilt capability in spite of this larger design depth of the gantry housing in the z-direction, the radially closed examination space must be enlarged, which leads to unnecessarily large radii and therewith high construction costs.